This invention relates to techniques for obtaining radiographic images and, more particularly, to an apparatus and method for obtaining improved images of opacified anatomy using a fluoroscopic type of equipment in conjunction with a video processor.
A typical x-ray fluoroscopy apparatus includes an x-ray source and an image intensifier which is used to detect the x-radiation. The output of the image intensifier is viewed by a television camera, and the resultant television signal can be presented on a monitor and/or recorded. When a body, such as that of a patient, is interposed between the x-ray source and the detector, x-rays are absorbed in varying degrees depending upon the thickness and composition of different regions of the body. This results in the presentation of a two-dimensional image that can be used, for example, for diagnosing structural abnormalities within the body.
The ability to "see" structure in the body using the described technique depends on the x-ray absorption properties of the structure of interest in relation to the x-ray absorption properties of the material(s) adjacent to the structure. The greater the difference, the greater the "contrast" the structure of interest will have in the resulting television image. The greater the contrast, the greater the clarity of the structure in the image. Consequently, achieving high contrast is a desirable quality with this imaging procedure.
Radiographic contrast agents are used to create a large difference in x-ray absorption behavior where little or none previously existed. For example, blood vessels are virtually invisible on fluoroscopic images (except in the chest) because blood, muscle, fat and soft tissue all possess similar x-ray absorption behavior. Radiographic contrast agents contain material (e.g. air, barium, iodine) which has x-ray absorption properties dissimilar to blood, muscle, fat and the soft tissue. For example, when a bolus of iodinated liquid contrast material is injected into an artery or vein, the vascular structure is given artificially higher contrast on an x-ray image while the contrast material is present within a certain vascular segment.
Digital video processing has been previously employed to improve radiographic imaging. Successful prior art digital processing techniques for image contrast enhancement have taken advantage of a priori knowledge of the time course behavior of radio-opaque contrast agents. By isolating and imaging the flow of such contrast agents through preselected arteries and organs within the body, both anatomical and physiological information related to organ function have been obtained without the need for selective catherization and its attendant risks.
So-called mask-mode imaging is a straightforward form of time-dependent subtraction imaging implemented by digital processing. A patient is placed on an x-ray table and a region of interest is chosen for study, e.g., the carotid arteries, or a heart chamber. A small needle is placed in an arm vein, through which 30-50 ml of iodinated contrast agent later is typically injected. Prior to injection, a single digital image is formed over several video frames (typically one to four) and stored in a digital memory. The contrast agent then is injected rapidly (e.g. in three to five seconds) and flows to the right heart, then to the lungs and to the left heart chambers from where it is pumped throughout the body's arterial system. As the contrast material passes through the region of interest, a sequence of additional images is accumulated within a second digital memory. Each of these post-opacification images is subtracted sequentially from the preinjection image. Subtraction images formed in this way have been synthesized at about one image per second for relatively stationary arterial structures. For rapidly moving cardiac structures the rate has been increased to about fifteen to sixty images per second.
The time-dependent subtraction images thus formed ideally would display only opacified cardiovascular anatomy. In the absence of patient motion, image contrast due to unopacified anatomy, e.g. bones, is removed. The removal of extraneous image information permits contrast enhancement of the opacified structures. Once enhanced, the subtraction images are reconverted to video format and stored, e.g. on a video tape or a video disc. The entire processing and external storage can proceed in real-time with the patient on the table.
Another existing technique, which has been used for imaging the rapidly moving heart, is called time interval difference or "TID" imaging. TID images are formed sequentially from contiguous pairs of images. The technique can be thought of as a special type of mask mode imaging in which the mask image is continually updated. In the case of cardiac imaging, the time interval chosen is short (such as one-fifteenth of a second) and the image sequence approximates the first time derivative of the cardiac motion on a point-by-point basis. Slowly varying motion (patient movement or respiratory motion) is muted and the outlines of the ventricular borders are displayed as black silhouettes during contraction and white silhouettes during expansion.
One can analyze the way in which the mask mode and TID imaging techniques isolate image contrast by considering the temporal frequency response in each technique. In both cases a specific subset of the total dynamic image information is isolated by altering the temporal response of the imaging technique to preserve as much as possible the temporal frequency content of the desired information, while at the same time rejecting temporal frequency components outside of this range.
Consider, as illustrated in FIG. 1, the mask mode response for two points of a body, A and B, in the image plane. One of the points (A) lies over a region containing a peripheral artery and the other (B) lies over a region which does not. The patient is assumed to remain motionless. The temporal frequency spectrum F.sub.B (.omega.) associated with stationary anatomy contains information only at zero temporal frequency, since there is no variation of this point with time. F.sub.A (.omega.) contains information at zero temporal frequency, as was the case for F.sub.B (.omega.), as well as smaller components peaking in the vicinity of 1 to 3 Hz, a result of cardiac motion, and components peaking in the vicinity of 0.1 to 0.3 Hz, associated with the arrival and washout of the contrast bolus during a period of several seconds. The mask mode imaging response for region B therefore is zero and for region A the arterial opacification is displayed. The pulsatile arterial motion associated with periodic cardiac contractions is also visible, but aliased to lower frequencies, a result of imaging at only once per second. In this example, then, the particular temporal response of mask mode imaging is responsible for the isolation of the low contrast arterial structures.
In actual imaging situations, some patient motion usually is present. In the presence of such motion, nonzero temporal frequency components not associated with the passage of the contrast bolus through arterial structures are detected with mask mode imaging and can obscure the detection of arterial structures. Although post-processing a mask mode sequence by choosing an alternative mask often can compensate for patient motion, all too often patient motion results in a less than optimal image and occasionally results in an unuseable image sequence.
One also can analyze the way in which TID imaging isolates cardiac motion from slower varying respiratory motion. The temporal frequency response of TID is obtained by evaluating the Fourier transform ("FT") of .delta.f/.delta.t since the TID technique approximates the first time derivative on a point by point basis. The Fourier transform has the property that FT (.delta.f/.delta.T)=-2.pi.i.omega.{F(.omega.)}. The factor i indicates a 90.degree. phase shift. The important aspect is that the temporal frequency spectrum F(.omega.) is weighted by the frequency .omega.. This implies that low temporal frequency variations (respiration and motion) are suppressed and zero frequency components are eliminated relative to higher frequency variations (cardiac contraction). The frequency response of TID imaging at a rate of 15 Hz is shown in FIG. 2.
It is an object of the present invention to provide improvement over existing techniques for obtaining processed images of the internal structure of a body.